1. Filed of the Invention
The present invention relates to a wavelength tunable light source used in the field of light-coherent communication/measuring techniques or the like.
2. Description of the Related Art
Referring to FIG. 6, a wavelength tunable light source according to a related art will be described.
In FIG. 6, reference numeral 1 denotes a semiconductor laser (hereinafter abbreviated to LD), reference numeral 2 denotes a diffraction grating, reference numeral 3 denotes a mirror, reference numerals 5, 6, and 7 denote lenses, reference numeral 8 denotes an optical isolator, reference numeral 11 denotes an optical fiber, and reference numeral 21 denotes a motor.
An anti-reflection film is applied onto one end surface 1a (diffraction grating 2 side end surface) of the LD 1 to avoid Fabry-Perot resonance between the end surface 1a and the other end surface 1b of the LD 1. A light beam emitted from the end surface 1a applied the anti-reflection film is converted into a collimated light beam by the lens 6 and then the collimated light beam is incident on the diffraction grating 2. Then, among the light beams incident on the diffraction grating 2, only a light beam having a wavelength selected by a wavelength selection portion constituted by a combination of the diffraction grating 2 and the mirror 3 returns to the LD 1. That is, the end surface 1b of the LD 1 and the mirror 3 form an external resonator to make laser oscillation with the wavelength selected by the wavelength selection portion.
On the other hand, a light beam emitted from the other end surface 1b of the LD 1 is converted into a collimated light beam by the lens 5 to pass through the optical isolator 8. The collimated light beam is condensed by the lens 7 to be taken out as an output light beam by the optical fiber 11. In the wavelength tunable light source, the mirror 3 itself is rotated by the motor 21 so that the wavelength which is to be selected by the wavelength selection portion, that is, the wavelength which is subjected to laser oscillation is changed.
In the wavelength tunable light source shown in FIG. 6, however, mode hopping (instantaneous wavelength jumping caused by hopping of a longitudinal mode of a resonator into an adjacent longitudinal mode) occurred when the wavelength was tunable. Hence, continuous wavelength scanning could not be made. This caused a problem that a long time was required for measuring various kinds of characteristics concerning wavelength such as wavelength loss characteristic and the like.
For example, a Littman-configuration wavelength tunable light source shown in FIGS. 7 and 8 is known as a wavelength tunable light source to solve the aforementioned problem. In the wavelength tunable light source, the center of rotation of a mirror 3 is disposed at a specific point (where the resonator length changes in accordance with the selected wavelength so that the order of the longitudinal mode does not change when the mirror 3 is rotated to tune the selected wavelength) to suppress the occurrence of mode hopping. In the Littman-configuration wavelength tunable light source, in order to increase wavelength tunable resolution, there is generally used a system in which the mirror 3 is attached to a rotary arm 22 having the center of rotation at the aforementioned specific point and a position (a forward end portion of the rotary arm 22) at a distance of tens of millimeters to 100 mm from the center of rotation of the mirror 3 is pressed by a direct-drive motor 23.
In the Littman-configuration wavelength tunable light source shown in FIGS. 7 and 8, however, backlash, stick slip, or the like, occurred in screws or gears used in the inside of the direct-drive motor 23. For this reason, there was a problem that even when the rotation shaft of the motor portion rotated, the necessary mirror 3 could not rotate or might be worse in precision of reproducibility.
Moreover, when a stepping motor was used as the motor portion of the direct-drive motor 23, torque fluctuation or velocity fluctuation (see FIG. 3(b)) was theoretically generated in every basic steps. Hence, it was difficult to estimate wavelength (position) information in wavelength scanning.
The necessity of measuring characteristics of various kinds of optical parts such as wavelength loss characteristic more speedily and more accurately has risen with the advance of the popularization of dense wavelength division multiplexing (DWDM) in the field of optical communication in recent years. Defective accuracy in wavelength or defective reproducibility caused by such slight torque or velocity fluctuation in wavelength scanning has become a subject of discussion.